As to a structure of a DRAM having a stack capacitor, it is disclosed for example, in JP-A-1-119054 . The method of manufacturing the known DRAM will be described with reference to FIGS. 4A-4D.
After an element isolation film 102 and a gate electrode 104 are formed on a surface of a silicon semiconductor substrate 101, source/drain diffused layers 113 are formed selectively (FIG. 4A). Next, an interlayer insulating film 105 is formed on the whole surface of the silicon semiconductor substrate so as to cover the gate electrode 104, the diffused layers 113 and the isolation film 102, and a contact hole 106 reaching one of source and drain diffused layers is formed through the interlayer insulating film 105 ( FIG. 4B ). Then, a lower electrode 107, a dielectric film 108 and an upper electrode 109 of a capacitor for storing electric charges are formed successively at an area over the contact hole 106 (FIG. 4C), and then an interlayer insulating film 110 is formed to cover them.
Lastly, a contact hole 111 is formed through the interlayer insulating film 110 and the interlayer insulating film 105 to reach the other of the source and drain diffused layers 113, and a metallic interconnection 112 is formed to extend into the contact hole 111 (FIG. 4D).
One memory cell having one transistor and one capacitor in a DRAM is formed by the process described above. A write word line and a read bit line are provided by the electrode 104 and the metallic interconnection 112, respectively.
In a semiconductor memory device manufactured by a conventional method as described above, a lower electrode (storage node) constituting a capacitor in a DRAM is formed of a thin polysilicon film containing impurities. Therefore, when .alpha.-rays are incident to a Si substrate from cosmic rays in the space or radioactive substances such as Uranium contained in a small quantity in a package, electron-hole pairs are generated by ionization along an incident path of .alpha.-rays in a depletion layer at a depth of the Si substrate. These electron-hole pairs are accelerated by an electric field as applied to the depletion layer, amplified by an avalanche phenomenon and trapped by the lower electrode of the capacitor, thereby to change the capacitance of the capacitor. This invites a problem of a so-called soft error, resulting in less reliability.